Numerous industries utilize evaporation-vapor deposition systems to coat substrates with desired materials. Illustrative of such applications are U.S. Pat. Nos. 4,290,384, 4,842,893, 4,490,774, 4,325,986, and 4,543,275. Specifically, organic compounds are utilized as coatings in applications as diverse as protective materials, product packaging, informational displays, and electronic devices.
The process of depositing a thin layer of organic material onto a substrate consists of three major steps. First, the compound must be evaporated. Second, the organic vapor must be transported to the substrate, and third, the vapor must be condensed onto the substrate. The evaporation step, however, is in inherent conflict with the final condensation step. Easily evaporated organic compounds are hard to condense while hard to evaporate compounds are easier to condense. Because the condensation process is influenced by numerous processes that are difficult to control, efforts have concentrated on developing evaporator technology that can vaporize organic compounds at a low enough temperature to enhance the probability of condensation.
To meet the requirements of many industrial applications, an evaporator-vapor deposition apparatus and method should be capable of employing a wide range of liquids, operating at low temperatures and pressures to avoid material polymerization or degradation, achieving a wide range of mass flow rates, operating continuously for substantial periods of time without requiring maintenance, and achieving a uniform deposition of material on the target substrate.
Existing methods and devices for evaporating and depositing organic compounds on a substrate include ultrasonic evaporators, as shown in U.S. Pat. No. 4,842,893, and spinning disk/heated wall evaporators, as shown in U.S. Pat. No. 4,954,371.
Ultrasonic evaporators are typically comprised of a liquid delivery system supplying liquid through a liquid dispenser into the center of a horn shaped structure. The horn shaped structure flares into a cone shape at one end and is positioned within an enclosed evaporation chamber with heated walls. Liquid from the dispenser is drawn through an angle of 120 degrees and onto the cone area by capillary action. The horn undergoes a sinusoidal acceleration, exerts a force on the liquid, and induces a sinusoidal pressure variation as it accelerates the liquid. The degree of acceleration is dependent upon the effective coupling strength of the solid surface of the horn to the liquid on its surface. The liquid, accelerated by the horn motion, separates into smaller droplets which leave the horn, impinge upon the heated evaporator walls, and vaporize.
Problems occur, however, when the ultrasonic evaporation process is attempted in low pressure environments. At atmospheric pressure, the coupling strength between the solid horn surface and liquid is sufficient to enable acceleration and droplet formation. As pressures decrease, the more volatile components of the liquid vaporize causing the formation of vapor pockets at the interface between the liquid and horn surface, the decoupling of the liquid from the horn surface, and, consequently, the inhibition of droplet formation. Larger droplets therefore impinge the evaporator wall and, because of the temperature gradient and increased temperature in the droplet interior, the liquid polymerizes. Under low pressure environments, ultrasonic evaporators can only be operated a few hours before cleaning the polymerized liquid is required. Because this is an inherent consequence of the ultrasonic evaporator design, ultrasonic evaporators can only be operated in higher pressure environments, rendering them unsuitable for low pressure vapor formation.
Conventional spinning disk/heated wall evaporators employ a capillary feeder to deliver liquid droplets to a rotating disk which then accelerates the droplets against the interior of a heated container, causing the droplets to evaporate. While the droplets when ejected off the surface of the spinning disk are small, they are unable to evaporate before subsequent drops are ejected and impinge on the same evaporator wall surface area. The result is a ring of polymerized or decomposed material that builds as a barrier between newly ejected droplets and the heat source. During operation, this system rapidly degenerates and requires frequent cleaning. This is a natural consequence of the limited surface area against which the ejected droplets may contact the heated wall. Solutions would require increasing the contact surface area through, possibly, constant vertical adjustment of the spinning disk to avoid consecutive ejectment of liquid material to the same wall area.
Both conventional spinning disk and ultrasonic evaporators have disadvantages which inhibit their effective use for evaporation of organic material in low pressure environments. Both operational schemes are subject to material flow variations which cause coating thickness variations on the substrate being coated. Currently evaporators such as conventional spinning disk and ultrasonic evaporators attempt to generate a constant gas flow rate by feeding liquid material into the apparatus at a fixed volumetric flow rate through the use of a positive displacement pump. Complications arise, however, due to the low pressure in the evaporator which produces low pressure in the feed tube. Because desirable organic liquids in their commercial form are a blend of compounds with varying vapor pressures, the low pressure in the feed tube causes certain high vapor pressure components to vaporize in the feed tube prior to delivery into the evaporator, thereby creating a vapor pocket within the feed tube, forcing the rapid expulsion of liquid situated in front of the vapor pocket, and providing no liquid flow when the vapor pocket exits the feed tube. To minimize this flow variation, prior art evaporators employ feed systems which incorporate a length of capillary tubing immediately between the feed tube and the evaporator environment. The capillary tubing provides viscous frictional resistance to the liquid flow, thereby sustaining pressure in the feed tube. This solution, however, does not completely eliminate the existence of low pressure in the capillary tubing. Consequently, certain organic liquid components still tend to evaporate prior to exiting the capillary tubing, generating vapor pockets in the capillary tubing and causing a pulsation of liquid flow and pressure pulsation in the evaporator. The alternate release of liquid and vapor causes variations in coating thickness on the substrate.
Additionally, because ultrasonic and conventional spinning disk evaporators have a common dependence on capillary action to deliver the requisite liquid to the evaporator apparatus, the viscosity of the liquid being evaporated is limited to that of a liquid having a viscosity less than 100 cps. When the viscosity rises above 100 cps, the pressure required to drive the liquid out of the capillary tubing becomes very high. The increased pressure raises the shear stress on the liquid thereby initiating polymerization in the capillary tubing and, consequently, rapidly leading to clogging of the tubing. To dispense fluids that have a room temperature viscosity above 100 cps is to heat the liquid. Heating, however, makes the liquid susceptible to polymerization and, therefore, could also result in clogging.
Finally, because of the rapid build-up of polymerized or degraded organic material, both ultrasonic and conventional spinning disk devices have short operation times. Short operational times are inefficient and adversely impact productivity.